1. Field of the Invention
The present invention relates to an organic electroluminescence device (hereinafter abbreviated as organic EL device). In particular, the present invention relates to an organic electroluminescence device including a red emitting layer.
2. Description of Related Art
An organic EL device, which includes an organic thin-film layer (in which an emitting layer is included) between an anode and a cathode, has been known to emit light using exciton energy generated by a recombination of holes and electrons that have been injected into the emitting layer.
Such an organic EL device, which has the advantages as a self-emitting device, is expected to serve as an emitting device excellent in luminous efficiency, image quality, power consumption and thin design.
An example of a further improvement made in an organic EL device is an improvement in luminous efficiency.
In this respect, in order to enhance internal quantum efficiency, developments have been made on an emitting material (phosphorescent material) that emits light using triplet excitons. In recent years, there has been a report on a phosphorescent organic EL device.
Since the internal quantum efficiency can be enhanced up to 75% or more (up to approximately 100% in theory) by forming the emitting layer (phosphorescent-emitting layer) from such a phosphorescent material, an organic EL device having high efficiency and consuming less power can be obtained.
In forming the emitting layer, a doping method, according to which an emitting material (dopant) is doped to a host material, has been known as a usable method.
The emitting layer formed by the doping method can efficiently generate excitons from electric charges injected into the host material. With the exciton energy generated by the excitons being transferred to the dopant, the dopant can emit light with high efficiency.
In order to intermolecularly transfer the energy from the host material to the phosphorescent dopant, excited triplet energy EgH of the host material is required to be larger than excited triplet energy EgD of the phosphorescent dopant.
A known representative example of a material having effectively-large excited triplet energy has been CBP (4,4′-bis(N-carbazolyl)biphenyl). See, for instance, Document 1: US2002/182441.
By using CBP disclosed in Document 1 as the host material, energy can be transferred to a phosphorescent dopant for emitting light of a predetermined emitting wavelength (e.g., green, red), by which an organic EL device of high efficiency can be obtained.
However, although an organic EL device in which CBP is used as the host material exhibits much higher luminous efficiency due to phosphorescent emission, the organic EL device has such a short lifetime as to be practically inapplicable.
Such a problem is considered to be attributed to considerable degradation of molecules by holes due to not-high oxidation stability that the molecular structure of CBP exhibits.
Alternatively, Document 2 (WO2005/112519) discloses a technique according to which a condensed-ring derivative containing a nitrogen-containing ring such as carbazole is used as the host material for a red-phosphorescent-emitting layer. Although the technique disclosed in Document 2 contributes to an improvement in the luminous efficiency and lifetime, the improved luminous efficiency and lifetime are not always sufficient for practical application.
On the other hand, a variety of host materials (fluorescent hosts) for fluorescent dopants are known. Various proposals have been made on a host material capable of, with a combination of a fluorescent dopant, providing a fluorescent-emitting layer excellent in luminous efficiency and lifetime.
However, although a fluorescent host has larger excited singlet energy Eg(S) than a fluorescent dopant, such a fluorescent host does not necessarily have larger excited triplet energy Eg(T). Accordingly, it is not successful to simply apply the fluorescent host to the host material (phosphorescent material) for a phosphorescent-emitting layer.
A well-known example of such a fluorescent host is an anthracene derivative.
However, excited triplet energy Eg(T) of an anthracene derivative is relatively small (approximately 1.9 eV). Thus, energy cannot be reliably transferred to a phosphorescent dopant for emitting light having a wavelength in a visible light range of 520 nm to 720 nm. In addition, excited triplet energy cannot be trapped within the emitting layer.
Accordingly, an anthracene derivative is not suitable for the phosphorescent host.
Further, derivatives such as a perylene derivative, a pyrene derivative and a naphthacene derivative are not preferable phosphorescent hosts for the same reason above.
Alternatively, an exemplary arrangement in which an aromatic hydrocarbon compound is used as the phosphorescent host has been known (Document 3: JP-A-2003-142267). In such an arrangement, a compound in which two aromatic groups are bonded as substituents to a benzene central skeleton in meta positions is used as the phosphorescent host.
However, since the aromatic hydrocarbon compound disclosed in Document 3 is structured such that molecules extend from the benzene central skeleton in a manner symmetrical relative to the benzene central skeleton, an emitting layer in which the aromatic hydrocarbon compound is used tends to be easily crystallized.
In addition, Patent Document 4 (WO2007/046658), Document 5 (JP-A-2006-151966), Document 6 (JP-A-2005-8588), Document 7 (JP-A-2005-19219), Document 8 (JP-A-2005-197262) and Document 9 (JP-A-2004-75567) disclose organic EL devices in which various aromatic hydrocarbon compounds are used. However, none of the above documents refers to effectiveness of aromatic hydrocarbon compounds as the phosphorescent hosts.
As described above, no host material has been known to be capable of efficiently transferring energy to the phosphorescent material while exhibiting such a long lifetime as to be practically applicable, which has hindered a practical realization of a device in which a phosphorescent material is used.